PUBLICATION 8167 SMALL GRAIN PRODUCTION MANUAL PART 4
Fertilization of Small Grains
UNIVERSITY OF
CALIFORNIA
Division of Agriculture
and Natural Resources
http://anrcatalog.ucdavis.edu
Doug Munier, University of California Cooperative Extension Farm Advisor, Glenn County;
Tom Kearney, University of California Cooperative Extension Farm Advisor Emeritus,
Yolo-Solano Counties; G. S. Pettygrove, Extension Specialist, Soils, Department of Land,
Air, and Water Resources, University of California, Davis; Kent Brittan, University of
California Cooperative Extension Farm Advisor, Yolo-Solano-Sacramento Counties; Marsha
Mathews, University of California Cooperative Extension Farm Advisor, Stanislaus County;
and Lee Jackson, Extension Specialist, Small Grains, Department of Plant Sciences,
University of California, Davis
This publication, Fertilization of Small Grains, is the fourth in a fourteen-part series of
University of California Cooperative Extension online publications that comprise the
Small Grain Production Manual. The other parts cover specific aspects of small grain
production practices in California:
•Part 1: Importance of Small Grain Crops in California Agriculture,
Publication 8164
•Part 2: Growth and Development, Publication 8165
•Part 3: Seedbed Preparation, Sowing, and Residue Management,
Publication 8166
•Part 5: Irrigation and Water Relations, Publication 8168
•Part 6: Pest Management—Diseases, Publication 8169
•Part 7: Pest Management—Insects, Publication 8170
•Part 8: Pest Management—Vertebrates, Publication 8171
•Part 9: Pest Management—Weeds, Publication 8172
•Part 10: Small Grain Forages, Publication 8173
•Part 11: Small Grain Cover Crops, Publication 8174
•Part 12: Small Grains in Crop Rotations, Publication 8175
•Part 13: Harvesting and Storage, Publication 8176
•Part 14: Troubleshooting Small Grain Production, Publication 8177
Nitrogen fertilizer is usually needed every season by small grain crops. The amount
needed depends on soil type, previous crop, rainfall, irrigation, projected yield, and
quality goals. Under some circumstances, it may be necessary to apply phosphorus,
potassium, sulfur, or zinc; perform soil tests before sowing to determine whether fertilizers are needed. The nitrogen status of a small grain crop is best monitored during
the season using tissue analysis. Since tillers and heads are formed in the very early
growth stages, adequate nutrient levels in the root zone at this time are important for
attaining maximum yield. Careful soil sampling and analysis are necessary for determining soil nutrient levels and fertilizer requirements.
Soi ls and soil fer ti lity
Small grain crops are grown on many soil types in California. Soil textures vary from
gravel to clay to the Sacramento–San Joaquin Delta’s peat soils. Crop rooting depths
vary with soil texture, soil profile development, drainage, and the presence of restrict-
ANR Publication 8167
ing layers. Roots of small grains can penetrate 7 feet (2.1 m) deep on well-drained
deep soils if there are no restricting layers, but penetration of 3 to 5 feet (0.9 to
1.5 m) at maturity is more common. Soils in many rainfed areas are shallow claypans
or hardpans. Nitrogen loss can be very high on these soils during high-rainfall years
since denitrification occurs under waterlogged soil conditions. Nitrogen losses can be
minimized on shallow or coarse-textured soils by making split applications of
nitrogen.
NI T ROGEN
Role of Nitrogen in the Plant
Adequate nitrogen stimulates vegetative growth and increases yield and protein content. Excessive nitrogen increases lodging, delays maturity, increases the severity of
some diseases, contributes to groundwater pollution, and causes rainfed crops to
deplete available moisture too early in the season. The nitrogen requirement of the
crop is directly related to the final grain yield. Plants obtain nitrogen from residual
nitrogen in the soil, from nitrogen released from decaying organic matter (including
the residue of the previous crop), and from applied fertilizer. On most soils 100 to 180
pounds per acre (112 to 202 kg/ha) of nitrogen may produce 3 to 3.5 tons per acre
(6.7 to 7.8 t/ha) of wheat grain, depending on the previous crop, winter soil moisture,
and rainfall conditions. Durum wheat may require higher rates of nitrogen, up to 240
pounds per acre (269 kg/ha). Barley and oat require less nitrogen, with optimal yields
attained with applications of 50 to 120 pounds per acre (56 kg/ha).
Nitrogen Deficiency Symptoms
Nitrogen deficiency symptoms are characterized by an overall yellowing of leaves, beginning with the bottom (older) leaves. Younger leaves remain green and appear healthy.
Plants are smaller and produce fewer tillers than plants with adequate nitrogen.
Nitrogen Requirements and Rates
The amount of nitrogen required by the crop depends on the type of small grain,
the previous crop in the rotation, the soil type, and weather conditions and cultural
practices during the growing season. Barley, oat, triticale, and wheat require different
amounts of nitrogen; the amount depends on the yield potential of the crop and the
intended use (grain production requires higher nitrogen levels than forage production). More nitrogen is required when wheat follows crops such as rice, cotton, or
wheat than when wheat follows vegetable crops, since more residual nitrogen normally
remains after the harvest of vegetables. Large amounts of residue from any previous
crop may require more nitrogen at sowing to provide enough available nitrogen for
small grain growth and residue breakdown. Nitrogen may be lost on gravelly or sandy
soils due to leaching below the root zone. Losses can occur from waterlogged soils
when nitrogen is lost to the atmosphere as nitrogen gas (N2) or nitrogen dioxide (N2O)
through a biological process called denitrification. Waterlogging is likely on heavy
soils and/or soils with a hardpan or claypan. Excessive winter rains and excess irrigation can cause nitrogen loss from any soil. Sowing on raised beds rather than on flat
ground provides better drainage, reduces nitrogen loss, and provides better soil conditions for root growth.
If the crop is sown in late fall and makes little growth before the onset of cold
weather, little nitrogen uptake will occur during winter. In the Central Valley, a substantial amount of nitrogen uptake normally occurs beginning in late January to early
February. The rate of accumulation increases through the spring, peaking at about
flowering and then decreasing as the crop reaches maturity.
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Rates for rainfed production
Three types of cropping are common in rainfed small grain production: 3-year or longer rotations (pasture-fallow-small grain), 2-year rotations (fallow-small grain), and
annual cropping. Less nitrogen is applied under 3-year rotations than under annual
cropping because nitrogen accumulates in soils during pasture and fallow years. Fields
that produce abundant annual clovers require less applied nitrogen if annual clovers
are pastured and plowed under as green manure crops. As little as 10 to 20 pounds
per acre (11.2 to 22.4 kg/ha) of nitrogen at sowing is sufficient if legumes are plowed
under as a summer fallow. If the fallow green manure crop consists entirely of grasses
or broadleaf weeds, an application of 20 to 40 pounds per acre (22.4 to 44.8 kg/ha) of
nitrogen is recommended at sowing. Residual soil fertility is adequate for optimal yield
for some long-term rainfed rotation acreage.
Since moisture is generally the limiting factor for rainfed yield, nitrogen rates
should be adjusted for lower yield potential, rainfall patterns, and soil moisture holding capacities. All nitrogen normally is applied preplant for rainfed production.
Split applications
Split applications of nitrogen are usually beneficial for irrigated production, although
all nitrogen can be applied at once during sowing on well-drained soils not normally
subject to leaching and waterlogging. When splitting the nitrogen on heavy or claypan
soils, apply one-half to two-thirds of the nitrogen at sowing and apply the remainder as
a topdressing. On extremely gravelly or sandy soils or on poorly drained soils of high
clay content, split the nitrogen application with half applied preplant and half topdressed to reduce nitrogen losses. On peat soils, apply about half the amount of nitrogen normally used on mineral soils; no preplant nitrogen is required, but a low rate of
nitrogen should be part of the starter fertilizer (high phosphorus content) at sowing.
Topdressing for yield
When topdressing, make one or two applications of 30 to 50 pounds per acre (33.6 to
56 kg/ha) of nitrogen, depending on the amount of rain during the winter. Nitrogen
applications made during the tillering stage, followed by rain or an irrigation, are most
effective for attaining maximum grain yield; applications made as late as boot stage
are less effective, while applications made at heading or later have little affect on grain
yield. If rain or irrigation occurs within a few days after application, little nitrogen is
lost to the air. If conditions are dry and cold (typical during tillering), losses are minimal if rain or irrigation occurs within 2 weeks. Volatilization of nitrogen to the air is
greatest under warm, moist conditions.
Stem nitrate-nitrogen (NO3-N) tissue tests are an effective way to monitor the
nitrogen status of the crop. Table 1 provides critical stem NO3-N levels for wheat and
barley as the crop develops from the third and fourth leaf stage to boot stage. These
tests are not effective for managing nitrogen fertility
after heading when the goal is to achieve high grain
Table 1. Critical stem NO3-N levels during vegetative growth for
wheat and barley
protein. Proper tissue sampling is important to attain
a valid, informative analysis. Collect 20 to 40 stems at
NO3-N (ppm dry weight)
random from typical areas of the field. Cut off the roots
Growth stage
Deficient
Adequate
Excessive
and plant tops and send the bottom 1 to 2 inches (2.5
3 to 4 leaf
< 7,000
7,000–12,000
> 12,000
to 5 cm) of the stems to the laboratory for analysis. Be
tillering
< 6,000
6,000–11,000
> 11,000
certain the stem tissue sample is not contaminated with
jointing
< 5,000
5,000–10,000
> 10,000
soil or leaves. Submit the tissue samples for analysis the
boot
< 4,000
4,000–9,000
> 9,000
same day they are collected.
ANR Publication 8167
Applications to improve grain quality
Wheat grown for bread flour should be managed to attain high bushel weight and a
grain protein content above 13 percent, as well as maximum grain yield. An irrigated
wheat crop with high yield potential should receive 100 to 180 pounds per acre (112
to 201.6 kg/ha) of nitrogen in a combination of preplant and topdressed (during tillering) applications. After heading, wheat may require one more nitrogen application of
20 to 50 pounds per acre (22.4 to 56 kg/ha) to produce a high protein content in the
grain. Nitrogen can best be applied during the 3 weeks from just after the spikes have
emerged from the flag leaf sheath to about 2 weeks after flowering. An application is
effective only when coordinated with an irrigation or rainfall. Low rates are appropriate for low-yielding crops, while higher rates are best suited for yields above 3 tons per
acre (6.7 t/ha).
Late-season application of nitrogen may not be needed to attain high grain protein content if the wheat grain yield potential is less than about 2.25 tons per acre (5.0
t/ha), and if nitrogen was applied preplant and topdressed during the tillering stage.
Cool, dry weather during grain filling generally results in higher grain yields; management for protein is more critical under those conditions.
Nitrogen application near the boot stage (before heading) typically increases the
grain protein content about 0.5 to 1.0 percent. The increase is not as large as when
nitrogen is applied at flowering, when nitrogen applications usually increase grain protein content by 1.0 to 1.5 percent or more.
Water-run Nitrogen
Water-run applications of anhydrous ammonia, UAN-32, aqua ammonia, or urea
applied at the beginning of an irrigation set are preferred for late-season fertilization.
This provides an effective means of applying the necessary nitrogen and water for
maximum yields. Foliar nitrogen applications are also effective at raising grain protein
content but are usually more expensive and may damage the leaves under many
conditions.
Forms and Costs of Nitrogen Fertilizers
The choice of fertilizer material depends on current weather conditions, weather forecasts, and fertilizer costs. Urea (46-0-0) is the highest-analysis nitrogen fertilizer available and is usually the least expensive dry form of nitrogen. It is particularly effective
when broadcast and followed by at least 0.5 inches (2.5 cm) of rain within 5 days after
application. Urea is converted to ammonium nitrogen and then to nitrate nitrogen by
soil microbial processes, so it is released over a longer period of time and is less prone
to leaching from the root zone than are some other forms of nitrogen. Urea is relatively
unstable when broadcast onto the soil surface, however, and volatilization (loss to the
air) can occur.
Ammonium nitrate (34-0-0) is an alternative to urea, if it is available, but it is
more expensive. It is preferable to urea when the crop is severely deficient because
both the nitrate and ammonium forms of nitrogen are readily taken up by the crop and
recovery is more rapid. Ammonium sulfate (21-0-0) containing 24 percent sulfur is
desirable if there is a likelihood of sulfur deficiency, but this material is usually more
expensive than urea. A blend of mostly urea with some ammonium sulfate is available
in some areas.
Applying Nitrogen in Dairy Lagoon Water
Dairy lagoon nutrient water (including liquid manure and wastewater) can supply
most or all of the nitrogen, phosphorus, and potassium needs of small grain forage
crops. However, these nutrients must be managed carefully. Misused, they can kill or
ANR Publication 8167
damage crops, contaminate ground and surface waters, and produce forage that is toxic
to animals consuming it. It is critical to understand the forms and concentrations of
nutrients and how they behave in the soil, and to apply the appropriate amount of nutrients during or just prior to the time the crop will take the nutrients up.
Forms of nitrogen in lagoon water
Lagoon water contains ammonium forms and organic forms of nitrogen. The
ammonium in lagoon water behaves the same as ammonium in fertilizers. Ammonium
is positively charged and adheres to soil particles, which are mostly negatively charged.
When lagoon water is applied, most of the nitrogen remains in the upper foot of the soil
because the ammonium adheres to the soil. As long as nitrogen is in the ammonium
form, it is resistant to leaching. However, ammonium is converted to nitrate by microorganisms in the soil. This process occurs rapidly when the soil is warm and slowly when
the soil is cold; ammonium applied in early fall on warm soils converts to nitrate within
days of application. Nitrate is negatively charged and leaches readily during irrigation or
heavy rainfall.
Organic nitrogen in lagoon water is bound up in particles of organic matter and
must be broken down into ammonium or nitrate by microorganisms before it becomes
available to the crop. The rate of breakdown depends on moisture content, soil temperature, and the resistance of the material to decay. The presence of organic nitrogen
complicates the use of lagoon water on crops because if a crop does not take up organic
nitrogen as it is released, the nitrogen may become a source of groundwater contamination. In determining rates of nitrogen fertilizer application, take into account the release
of available nitrogen from current and past applications of lagoon water and reduce
applications of available nitrogen accordingly.
Like ammonium, organic nitrogen also remains in the upper foot of soil because,
depending on the porosity of the soil, the particles do not move far through the soil. If
the lagoon water has a high level of solids, the largest particles may remain on the soil
surface as a crust until incorporated by tillage.
Application rates and timing
Winter forages planted during November in the Central Valley typically take up
less than 50 pounds per acre (56 kg/ha) of nitrogen in the aboveground portion of the
crop prior to mid-January. Uptake is higher under conditions of increased growth, such
as earlier planting or sustained unseasonably warm temperatures. Under normal circumstances, earlier-maturing winter forages begin to take up more significant amounts
of nitrogen starting in late January to early February. The rate of nitrogen uptake peaks
in March and April but continues until crop maturity. The nitrogen uptake pattern for
later-maturing cereal forages is similar to the early-maturing types, but peak uptake
occurs somewhat later than in the earlier types. Nitrogen applications should be timed
to provide available nitrogen during or immediately prior to the period when the crop
will use them.
Application rates of available nitrogen should not exceed the amount of uptake
expected between the time of application and the next anticipated heavy rainfall or
irrigation that exceeds the amount needed to refill the soil profile. A preplant irrigation
containing lagoon water in early fall may not meet the demands of spring growth, especially in years with heavy winter rains. The bulk of lagoon water should be applied in
late January to early February, with only light application in fall preplant irrigation. No
more than about 120 pounds (54 kg) of available nitrogen should be applied at any one
time; if more than this is needed, a second spring application should be made. If the fertilization history of the field includes dry manure or lagoon water containing significant
organic nitrogen, in many cases the organic nitrogen will supply all the nitrogen the
ANR Publication 8167
crop needs until the early spring. On heavier soils it may be necessary to plant on beds
to prevent crop waterlogging injury from midwinter lagoon water applications.
If it is necessary to apply lagoon water in the early fall, one way to use the nutrients is to sow cereal forages in late September or early October so that the crop will
make sufficient growth and take up the applied nitrogen before the onset of cold temperature that slows growth. This forage can be cut during the winter and allowed to
regrow during the spring. Forage yield and nitrogen uptake in such a system can be
higher than in a single-cut system. The major drawback to cutting during the winter is
that a period of dry weather is needed to dry the soil enough to allow access by equipment and to wilt the forage to a suitable moisture for ensiling. Sowing very early in the
fall and leaving the crop uncut can lead to lodging and disease problems.
Applying undiluted lagoon water in most cases results in excess nutrient applications that not only threaten groundwater but also may reduce yields due to waterlogging, salt burn, and lodging. If high amounts of nitrogen remain in the soil near harvest,
nitrates may accumulate in the crop to levels that are toxic to livestock.
In most cases, lagoon water must be blended with fresh water. This can be done by
using a flow meter and throttling valve on the lagoon pump output to mix the correct
amount of lagoon water into the irrigation water. Pipelines must be correctly sized to
prevent plugging from solids when reducing the flow rate of lagoon water. In addition
to installing a flow meter system, it may be necessary to increase lagoon capacity, install
additional pipelines, or make other modifications. Separators, settling basins, and other
technology that minimizes the buildup of solids in the pond are also important components of a lagoon nutrient management system. Changes to the irrigation system itself
may also be necessary, because if the irrigation is not uniform, all parts of the field may
not get the same amount of nutrient, resulting in excess nitrogen in some areas of the
field (usually the head end) and not enough in others.
P HOSP HORU S
Role of Phosphorus in the Plant
Phosphorus is a component of cell membranes and plays a role in the transfer and storage
of energy within plant cells. It makes up part of the structure of key molecules, including
DNA. Phosphorus nutrition is particularly important for seedling vigor, root development,
and early-season growth. Normal root and shoot growth and the rate of photosynthesis
are governed by phosphorus status. Phosphorus also has a regulating role in tillering, leaf
expansion, leaf size, and the rate of assimilate production per leaf area.
Phosphorus Deficiency Symptoms
Phosphorus deficiency is most likely on shallow upland (terrace and foothill) soils.
Symptoms of phosphorus deficiency include slow early growth, lack of tillering, and
sometimes a slight purpling of plants. As in nitrogen deficiency, symptoms appear first
on older leaves and advance to younger leaves as phosphorus deficiency becomes more
severe. Deficient plants usually mature later than normal plants.
Phosphorus Requirements and Rates
In California, small grains are generally grown when soil temperatures are low and
phosphorus availability is reduced. If phosphorus is needed, placement with or near
the seed is best. On mineral soils a soil test (sodium bicarbonate extraction method)
on samples taken to plow-layer depth can serve as a guide for phosphorus fertilization.
Responses to phosphorus application are likely if phosphorus levels are less than 6 ppm,
variable if phosphorus levels are from 6 to 15 ppm, and unlikely if phosphorus levels
are above 15 ppm. Many growers apply a low-nitrogen, high-phosphorus fertilizer, such
ANR Publication 8167
as 11-48-0, 11-52-0, 10-50-0, or liquid 10-34-0, at sowing time with or near the seed.
Monoammoniun phosphate, with an approximate 1:4 to 1:5 nitrogen to phosphorus
ratio, is more desirable than a diammoniun phosphate (1:3 ratio) because little, if any,
toxic ammonia is released. Urea, urea phosphate, or diammonium phosphate (18-46-0)
are more hazardous because the initial reaction in the soil releases ammonia that can kill
seedlings.
If a soil test indicates phosphorus deficiency, apply 30 to 40 pounds per acre (33.6
to 44.8 kg/ha) of P2O5 drilled with seed for irrigated crops, and 20 to 30 pounds per acre
(22.4 to 33.6 kg/ha) of P2O5 for dryland crops. To avoid injuring seed, no more than 25
to 30 pounds per acre (28 to 33.6 kg/ha) of nitrogen should be drilled. If phosphorus is
broadcast, use higher rates, up to 80 pounds per acre (89.6 kg/ha) of P2O5. Application
of nitrogen at rates greater than 10 to 15 pounds per acre (11.2 to 16.8 kg/ha) combined
with the higher rates of phosphorus stimulates the growth of grassy weeds.
The bicarbonate extraction method and phosphorus levels cited above are not reliable if small grains are sown directly after a crop of rice. An increased yield response to
phosphorus is nearly always expected following rice, particularly if rice has been grown
for several seasons.
Phosphorus applications are often needed on peat soils because phosphorus becomes unavailable when soil pH is low, which is typical in peat soils. In the
Sacramento–San Joaquin Delta, wheat yield increases of up to 800 pounds per acre
(896 kg/ha) can often be obtained by applying 50 pounds per acre (56 kg/ha) of P2O5
with the seed at planting. Phosphorus also becomes unavailable in very alkaline soils;
application rates on high-pH soils should be increased to 30 to 50 pounds per acre
(33.6 to 56 kg/ha) of P2O5 and drilled with the seed.
Phosphorus and Dairy Lagoon Water
Soils that receive large amounts of dairy manure, especially solids and liquid high in solids, can develop high levels of phosphorus. In areas of the Central Valley where there is
no surface runoff to carry away phosphorus particles, soils are heavier, and groundwater
is deep, no deleterious effects of excess phosphorus in soil have been demonstrated. Do
not overapply phosphorus in areas where soils are sandy, groundwater is shallow, and
water applied to fields enters waterways through tile drains or direct connection.
S UL FUR
Role of Sulfur in the Plant
Sulfur is an essential constituent of the amino acids cysteine, methionine (required for
protein synthesis), several coenzymes (e.g., biotin, co-enzyme A, thiamine pyrophosphate, and lipoic acid), thioredoxins, and sulfolipids. Sulfur is an important factor in
wheat bread-making quality (protein level, loaf volume, and loaf texture). Nitrogen and
sulfur requirements are closely linked since both are required for protein synthesis.
Sulfur Deficiency Symptoms
Sulfur-deficient plants become spindly and develop a pale yellow color. The symptoms
are very similar to nitrogen deficiency. Sulfur deficiency reduces the number of grains
per spike; the number of tillers and grain weight are less affected unless the deficiency
is severe.
Sulfur Requirements and Rates
Sulfur deficiency most often occurs on gravelly or sandy soils. It is more common during winter to early-spring periods when soils are cool and wet or waterlogged. Sulfur
enters the plant through the roots in the form of sulfate. Nitrogen metabolism and sul-
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fur metabolism are strongly interdependent: when sulfur is deficient relative to nitrogen,
nonprotein compounds such as amines accumulate, resulting in a nitrogen to sulfur (N:
S) ratio of greater than 15:1. Wheat is likely to be sulfur deficient if it has a total sulfur
of less than 0.20 percent and a N:S ratio greater than 17:1 in the upper fully developed
leaves at flag leaf to anthesis. The concentration of sulfur in grain and the N:S ratio have
been used to retrospectively diagnose sulfur deficiency, with critical values of 0.12 percent and 17:1 N:S.
Small grains have a lower sulfur requirement, 10 to 30 pounds per acre (11.2 to
33.6 kg/ha), than many other crops, but an adequate level of sulfur is necessary for satisfactory crop growth and for optimal levels of S-containing amino acids in grain. Sulfur
deficiency is best corrected by planting-time application of fertilizer that has nitrogen
and sulfate-sulfur, such as ammonium sulfate (21-0-0). Applications of relatively low
rates of readily available sulfate-sulfur sources can be effective corrective treatments during the active early spring growth period; elemental sulfur is much less effective. If elemental sulfur is used, applications must precede the growing season by sufficient time
(probably several months) to allow conversion of sulfur to the sulfate form for plant
use. Moisture is also required for this process. Gypsum is also an immediately available
but slow releasing form of sulfur. Sulfur deficiency symptoms disappear in most instances as soil temperatures warm, moisture levels drop below saturation, and plant growth
progresses.
P OTASS IUM
Role of Potassium in the Plant
Potassium is essential for plant growth and development. It activates enzymes needed
for growth and is necessary for the formation and transfer of starches, sugars, and oils;
the absorption of nutrients; and the efficient use of water. It enables plants to grow
strong roots and resist drought, winter-kill, and root diseases. It also helps develop
strong stems and decreases lodging.
Potassium Deficiency Symptoms
Potassium deficiency symptoms generally appear on the older leaves first. Depending on
the severity of the deficiency, the entire plant may be affected, and all leaves may have
an unthrifty, spindly appearance. During the early states of deficiency, the leaf tips and
margins are chlorotic. Necrosis appears on leaves under
severe deficiency as speckling along the length of the leaf
Table 2. Potassium sufficiency in wheat for selected
and spreads quickly to the tip and margins. An “arrow” of
growth stages
green tissue remains from the base upward to the center
Sufficiency range
of the leaf. Complete death of older leaves is common,
Growth stage
Plant part
(low to high)
and plants appear to have dried prematurely, as with
3 to 4 leaf
underground stem
2.0–3.0%
drought stress.
tillering
jointing
boot
whole plant
2.4–4.0%
aboveground stem,
lower 2 inches (5 cm)
2.0–3.0%
top 4 leaves
2.4–4.0%
aboveground stem,
lower 2 inches (5 cm)
1.5–2.7%
top 4 leaves
2.0–3.0%
aboveground stem,
lower 2 inches (5 cm)
1.0–2.4%
top 4 leaves
1.5–2.7%
Sources: California Plant Health Association 2002.
Potassium Requirements and Rates
In California, yield responses by small grains to applications of potassium are highly unusual and occur only on
the most deficient soils, such as soils with ammonium
acetate extractable potassium levels less than 60 ppm.
Plants need as much potassium as nitrogen during rapid
growth periods. Potassium sufficiency in wheat depends
on the stage of growth (table 2). Several sources of potassium (potash) are used as commercial fertilizers (table 3).
ANR Publication 8167
Table 3. Commercial fertilizer sources of potash
Formula
Water-soluble
potash (K2O) %
Other
nutrients
monopotassium phosphate
KH2PO4
32–34
50­–52% P2O3
potassium chloride (MOP)
KCl
60–62
—
K2SO4  ·  2MgSO4
22
18% S, 11% Mg, 0.1% Ca
KNO3
44–46
13% N
potassium sulfate (SOP)
K2SO4
50–53
18% S
potassium thiosulfate
K2S2O3
25
17% S
Fertilizer material
potassium magnesium sulfate
potassium nitrate
Potassium is rapidly
absorbed and very mobile, making it a good additive to foliar
fertilizers. The crop response to
foliar nutrition depends on the
soil, the crop, and environmental
conditions.
ZINC
Role of Zinc in the Plant
Zinc is an essential micronutrient
for crop growth. It is needed for the production of auxins, growth-promoting substances
that control the growth of shoots. A critical level of zinc is required in the soil for roots
to grow or function effectively.
Sources: California Plant Health Association 2002.
Zinc Deficiency Symptoms
Zinc deficiency is probably the most widespread micronutrient deficiency in small
grains. It can occur in cold and warm climates, acid and alkaline soils, and heavy and
light soils. In general, stems and leaves of deficient plants fail to develop to normal size,
and some of the tissues between leaf veins contain so little chlorophyll that they turn
yellow. The first symptoms of zinc deficiency normally appear on middle-aged leaves.
Initial symptoms include a change in leaf color from healthy green to muddy grey-green
in the central portion of the blade. Leaves appear drought-stressed, and necrotic areas,
beginning as small spots, develop and extend to the leaf margins. Leaves may take on
an oily appearance, and the necrotic areas may become large and surrounded by mottled
yellow-green areas. Zinc-deficient leaves eventually collapse in the middle regions.
Severe zinc deficiency can result in stunted, chlorotic plants with many collapsed leaves
due to necrosis in the center of the leaves.
Zinc Requirements
Zinc deficiency arises from a low content of zinc in soil, unavailability of zinc in highpH soil, or management practices that depress the availability of zinc. Although most
mineral soils contain 80 to 300 ppm of total zinc, DTPA extractable zinc is usually less
than 1 ppm; the remainder is fixed in an unavailable form. The unavailability of zinc
can be attributed to soil alkalinity: when the soil pH is above 7.0, zinc availability is
generally reduced. Zinc availability is low in some soils with high organic matter, in
some clay soils with high magnesium content (these soils fix zinc in an unavailable form
by strong adsorption on the clay minerals in place of magnesium), and in soils high in
phosphorus. Low zinc levels combined with high phosphorus levels enhance accumulation of phosphorus in old leaves to concentrations that are toxic; this enhances necrotic
symptoms in old leaves. Yield responses to applications of zinc occur only on the most
deficient soils, such as soils with zinc DTPA extractable levels below 0.3 ppm.
R eference s
Cakmak, I., H. Ekiz, A. Yilmar, B. Torun, N. Koleli, I. Gultekin, A. Alkan, and S.
Eker. 1997. Differential response of rye, triticale, bread and durum wheats to
zinc deficiency in calcareous soils. Plant and Soil 188:1–10.
California Plant Health Association. 2002. Western fertilizer handbook. 9th ed.
Upper Saddle River, NJ: Prentice Hall.
1 0 ANR Publication 8167
Clement, L., L. Jackson, T. Kearney, J. Orr, R. Sailsbery, and J. Williams. 1982.
Wheat production in the Sacramento Valley. Oakland: University of California
Division of Agricultural Sciences Leaflet 21323.
Cook, R., and R. Veseth. 1991. Wheat health management. St. Paul: APS Press.
Elliott, D., D. Reuter, G. Reddy, and R. Abbott. 1997. Phosphorus nutrition of spring
wheat (Triticum aestivum L.) 1. Effects of phosphorus supply on plant symptoms, yield, components of yield, and plant phosphorus uptake. Australian
Journal of Agricultural Research 48:855–867.
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1 1 ANR Publication 8167
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